Create external dependency for protofiles

The protofiles package installs .proto files for enterprise
support. This patch creates an external dependency target
that builds them into a static library.

BUG=None
TEST=emerge-BOARD platform2

Change-Id: I2298eba02435229b0d0bf00b3afbf30437a99451
Reviewed-on: https://chromium-review.googlesource.com/176694
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris Masone <cmasone@chromium.org>